Multi-wavelength light amplifier

ABSTRACT

A multi-wavelength light amplifier includes a first-stage light amplifier which has a first light amplifying optical fiber amplifying a light input, a second stage light amplifier which has a second light amplifying optical fiber amplifying a first light output from the first-stage light amplifier, and an optical system which maintains a second light output of the second-stage light amplifier at a constant power level. The first-stage and second-stage light amplifiers have different gain vs wavelength characteristics so that the multi-wavelength light amplifier has no wavelength-dependence of a gain thereof.

BACKGROUND OF THE INVENTION Field of the Invention

The present invention generally relates to a light amplifier for awavelength division multiplexed (WDM) optical transmission system, andmore particularly to a light amplifier having a two-stage configurationwhich eliminates a wavelength-dependence of the gain of the lightamplifier.

Recently, an optical communications network has increasingly been usedin practice. Nowadays, it is required that the optical communicationsnetwork cope with multi-media networking. A WDM system is moreattractive, particularly in terms of an increase in the transmissioncapacity. In order to realize the WDM system, it is necessary to use amulti-wavelength light amplifier capable of amplifying a wavelengthdivision multiplexed signal. It is required that such a multi-wavelengthlight amplifier does not have wavelength-dependence of the gain, whichis further required not to be changed due to a variation in the power ofthe input light.

A light amplifier is known which has an optical fiber doped with arare-earth element and directly amplifies the input light. There hasbeen some activity in the development of a multi-wavelength lightamplifier which amplifiers a wavelength division multiplexed lightsignal-including signal components having different wavelengths(channels).

However, normally, the rare-earth-element doped fiber amplifier has avery narrow range in which the gain thereof does not have thewavelength-dependence. In this regard, nowadays, there is no availablelight amplifier which can practically be used for the WDM system. Thatis, there is no available light amplifier which does not havewavelength-dependence of the gain, which is not changed due to avariation in the power of the input light. Particularly, thewavelength-dependence of the gain, which takes place when the inputpower changes, deteriorates the signal-to-noise ratio with respect to aparticular signal. This prevents the multi-wavelength light amplifierfrom being used in practice.

SUMMARY OF THE INVENTION

It is a general object of the present invention to provide amulti-wavelength light amplifier in which the above disadvantages areeliminated.

A more specific object of the present invention is to provide amulti-wavelength light amplifier which does not havewavelength-dependence of the gain, which is not changed due to avariation in the power of the input light.

The above objects of the present invention are achieved by amulti-wavelength light amplifier comprising: a first-stage lightamplifier which has a first light amplifying optical fiber amplifying alight input; a second-stage light amplifier which has a second lightamplifying optical fiber amplifying a first light output from thefirst-stage light amplifier; and an optical system which maintains asecond light output of the second-stage light amplifier at a constantpower level. The first-stage and second-stage light amplifiers havedifferent gain vs wavelength characteristics so that themulti-wavelength light amplifier has no wavelength-dependence of a gain.

The above multi-wavelength light amplifier may be configured as follows.The first-stage light amplifier comprises a first pump source whichpumps the first light amplifying optical fiber so as to have a firstgain vs wavelength characteristic in which as a wavelength of light tobe amplified becomes shorter, a gain of the first-stage light amplifierbecomes higher. The second-stage light amplifier comprises a second pumpsource which pumps the second light amplifying optical fiber so as tohave a second gain vs wavelength characteristic in which as a wavelengthof light to be amplified becomes longer, a gain of the first-stage lightamplifier becomes higher.

The above multi-wavelength light amplifier may be configured as follows.The first-stage light amplifier comprises a first pump source whichpumps the first light amplifying optical fiber so as to have a firstgain vs wavelength characteristic having a first linear gain slope. Thesecond-stage light amplifier comprises a second pump source which pumpsthe second light amplifying optical fiber so as to have a second gain vswavelength characteristic having a second linear gain slope. Acombination of the first and second-linear gain slopes results in a flatgain vs wavelength characteristic of the multi-wavelength lightamplifier.

The above multi-wavelength light amplifier may further comprise anoptical filter which emphasizes the gain vs wavelength characteristic ofthe first-stage light amplifier.

The above multi-wavelength light amplifier may further comprise anoptical filter which compensates for a difference between the gain vswavelength characteristics of the first-stage light amplifier and thesecond-stage light amplifier.

The above multi-wavelength light amplifier may be configured as follows.The optical filter is provided so as to follow the first-stage lightamplifier. The first-stage light amplifier comprises a first pump sourcewhich pumps the first light amplifying optical fiber so as to have afirst gain vs wavelength characteristic having a first linear gainslope. The second-stage light amplifier comprises a second pump sourcewhich pumps the second light amplifying optical fiber so as to have asecond gain vs wavelength characteristic having a second linear gainslope. The optical filter emphasizes the first linear gain slope toprovide an emphasized first linear gain slope. A combination of theemphasized first linear slope and the second linear gain slope resultsin a flat gain vs wavelength characteristic of the multi-wavelengthlight amplifier.

The above multi-wavelength light amplifier may be configured as follows.The optical filter is provided so as to follow the first-stage lightamplifier. The first-stage light amplifier comprises a first pump sourcewhich pumps the first light amplifying optical fiber so as to have afirst gain vs wavelength characteristic having a first linear gainslope. The second-stage light amplifier comprises a second pump sourcewhich pumps the second light amplifying optical fiber so as to have asecond gain vs wavelength characteristic having a second linear gainslope. The optical filter compensates for the difference between thefirst and second linear gain slopes so that a flat gain vs wavelengthcharacteristic of the multi-wavelength light amplifier can be obtained.

The above multi-wavelength light amplifier may be configured as follows.The first-stage light amplifier has a first AGC (automatic gain control)system so that a ratio of the input light and the first light output isconstant. The second-stage light amplifier has a second AGC system sothat a ratio of the first light output and the second light output isconstant.

The above multi-wavelength light amplifier may be configured as follows.The first-stage light amplifier has an AGC (automatic gain control)system so that a ratio of the input light and the first light output isconstant. The second-stage light amplifier has an automatic powercontrol (APC) system so that the second light amplifying optical fiberis pumped at a predetermined constant power level.

The above multi-wavelength light amplifier may be configured as follows.The first-stage light amplifier has an AGC (automatic gain control)system so that a ratio of the input light and the first light output isconstant. The second-stage light amplifier has an automatic levelcontrol (ALC) system so that the second light output is maintained at apredetermined constant power level.

The above multi-wavelength light amplifier may be configured as follows.The first AGC system comprises first means for detecting a first levelof the light input and a second level of the first light output andpumping the first light amplifying optical fiber so that a ratio of thefirst and second levels is maintained at a first predetermined constantvalue. The second AGC system comprises second means for detecting athird level of the first light output and a fourth level of the secondlight output and pumping the second light amplifying optical fiber sothat a ratio of the third and fourth levels is maintained at a secondpredetermined constant value.

The above multi-wavelength light amplifier may be configured as follows.The first-stage light amplifier has a first AGC (automatic gain control)system which detects a first amplified spontaneous emission of the firstlight amplifying optical fiber and pumps the first light amplifyingoptical fiber so that the first amplified spontaneous emission ismaintained at a first predetermined constant level. The second-stagelight amplifier has a second AGC system which detects a second amplifiedspontaneous emission of the second light amplifying optical fiber andpumps the second light amplifying optical fiber so that the secondamplified spontaneous emission is maintained at a second predeterminedconstant level.

The above multi-wavelength light amplifier may be configured as follows.The first-stage light amplifier has a first AGC (automatic gain control)system which detects a first pump light propagated through the firstlight amplifying optical fiber and pumps the first light amplifyingoptical fiber so that the first pump light is maintained at a firstpredetermined constant level. The second-stage light amplifier has asecond AGC system which detects a second pump light propagated throughthe second light amplifying optical fiber and pumps the second lightamplifying optical fiber so that the second pump light is maintained ata second predetermined constant level.

The above multi-wavelength light amplifier may be configured as follows.The first-stage light amplifier comprises a first pump source whichpumps the first light amplifying optical fiber through a first couplerso as to have a first gain vs wavelength characteristic in which as awavelength of light to be amplified becomes shorter, a gain of thefirst-stage light amplifier becomes higher. The second-stage lightamplifier comprises a second pump source which pumps the second lightamplifying optical fiber through a second coupler so as to have a secondgain vs wavelength characteristic in which as a wavelength of light tobe amplified becomes longer, a gain of the first-stage light amplifierbecomes higher. At least one of the first and second couplers has acharacteristic which emphasizes one of the gain vs wavelengthcharacteristics of the first-stage and second-stage light amplifiers.

The above multi-wavelength light amplifier may be configured as follows.The optical system which maintains the second light output of thesecond-stage light amplifier at a constant power level comprises avariable attenuator which is provided between the first-stage lightamplifier and the second-stage light amplifier and attenuates the firstoutput signal on the basis of the power level of the second lightoutput.

The above multi-wavelength light amplifier may be configured as follows.The optical system which maintains the second light output of thesecond-stage light amplifier at a constant power level comprises avariable attenuator which is provided so as to follow the second-stagelight amplifier and attenuates the second output signal on the basis ofthe power level of an attenuated second light output from the variableattenuator.

The above multi-wavelength light amplifier may be configured as follows.The optical system which maintains the second light output of thesecond-stage light amplifier at a constant power level comprises avariable attenuator which is provided between the first-stage lightamplifier and the second-stage light amplifier and attenuates the firstoutput signal on the basis of the power level of an attenuated-firstlight output from the variable attenuator.

The above multi-wavelength light amplifier may further comprise arejection filter which is provided between the first-stage lightamplifier and the second-stage light amplifier and prevents a pump lightwhich pumps the first light amplifying optical fiber from beingtransmitted to the second-stage light amplifier.

BRIEF DESCRIPTION OF THE DRAWINGS

Other objects, features and advantages of the present invention willbecome more apparent from the following detailed description when readin conjunction with the accompanying drawings, in which:

FIG. 1 is a block diagram of a multi-wavelength light amplifieraccording to a first embodiment of the present invention;

FIG. 2 is a diagram showing a principle of a multi-wavelength lightamplifier according to a second embodiment of the present invention;

FIG. 3 is a block diagram of a multi-wavelength light amplifieraccording to a third embodiment of the present invention;

FIG. 4 is a diagram showing a principle of the multi-wavelength-lightamplifier according to the third embodiment of the present invention;

FIG. 5 is a block diagram of a multi-wavelength light amplifieraccording to a fourth embodiment of the present invention;

FIG. 6 is a diagram showing a principle of the multi-wavelength lightamplifier according to the fourth embodiment of the present invention;

FIGS. 7A and 7B are diagrams showing a principle of a multi-wavelengthlight amplifier according to a fifth embodiment of the presentinvention;

FIG. 8 is a block diagram of a multi-wavelength light amplifieraccording to a seventh embodiment of the present invention;

FIG. 9 is a block diagram of a multi-wavelength light amplifieraccording to an eighth embodiment of the present invention;

FIG. 10 is a block diagram of a multi-wavelength light amplifieraccording to a ninth embodiment of the present invention;

FIG. 11 is a block diagram of a multi-wavelength light amplifieraccording to a tenth embodiment of the present invention;

FIG. 12 is a block diagram of a multi-wavelength light amplifieraccording to an eleventh embodiment of the present invention;

FIG. 13 is a block diagram of a multi-wavelength light amplifieraccording to a twelfth embodiment of the present invention;

FIG. 14 is a block diagram of a multi-wavelength light amplifieraccording to a thirteenth embodiment of the present invention; and

FIG. 15 is a block diagram of a multi-wavelength light amplifieraccording to a fourteenth embodiment of the present invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

FIG. 1 is a block diagram of a multi-wavelength light amplifieraccording to a first embodiment of the present invention. The amplifiershown in FIG. 1 includes a first-stage (front-stage) light amplifier 1and a second-stage (rear-stage) light amplifier 2. A variable attenuator(ATT) 11 is provided between the first and second amplifiers 1 and 2.The variable attenuator 11 is controlled by an automatic level control(ALC) circuit 14, which is controlled by a photodetector 13 such as aphotodiode. The photodiode 13 receives split light from a beam splittingcoupler 12, which follows the second-stage amplifier 2. An opticalsystem having a feedback loop is formed by the light splitting coupler12, the photodiode 13, the ALC circuit 14 and the variable attenuator11.

The first-stage amplifier 1 includes a first-stage light input monitormade up of a beam splitting coupler 3 ₁ and a photodiode 4 ₁, and afirst-stage light output monitor made up of a beam splitting coupler 3 ₂and a photodiode 4 ₂. Further, the first-stage amplifier 1 includes alight amplifying optical fiber 7 such as a rare-earth-element dopedoptical fiber and an exciting-light source (hereinafter referred to as apump source: PS) 9 ₁, which is controlled by an automatic gain control(AGC) circuit 6 ₁ provided in the first-stage amplifier 1. An AGC systemincluding the AGC circuit 6 ₁ and the above input and output monitorsperforms an AGC control of the pump source 9 ₁ so that the ratio of thelight input power level detected by the light input monitor and thelight output power level detected by the light output monitor can bemaintained at a constant value. The above ratio corresponds to the gainof the first-stage amplifier 1.

The second-stage amplifier 2 includes a second-stage light input monitormade up of a beam splitting coupler 3 ₃ and a photodiode 4 ₃, and asecond-stage light output monitor made up of a beam splitting coupler 3₄ and a photodiode 4 ₄. Further, the second-stage amplifier 2 includes alight amplifying optical fiber 8 such as rare-earth-element dopedoptical fiber, and a pump-source 9 ₂, which is controlled by an AGCcircuit 6 ₂ provided in the second-stage amplifier 2. An AGC systemincluding the AGC circuit 6 ₂ and the above input and output monitorsperforms a AGC operation of the pump source 9 ₂ so that the ratio of thelight input power level detected by the light input monitor and thelight output power level detected by the light output monitor can bemaintained at a constant value.

The combination of the first-stage amplifier 1 and the second-stageamplifier 2 functions to cancel the difference between the gain of theamplifier 1 and the gain of the amplifier 2 in each of the wavelengthsof the multiplexed signal. That is, the amplifiers 1 and 2 havedifferent gain vs. wavelength characteristics (which may be simplyreferred to as gain characteristics), which can be compensated by thecombination of the amplifiers 1 and 2. As a result, the entiremulti-wavelength light amplifier has a flat gain vs wavelengthcharacteristic.

It will now be assumed that G_(0,1) denotes an AGC control setting levelwhich causes the amplifier 1 to have a flat gain vs wavelengthcharacteristic in which the output spectra at the respective wavelengthsof the multiplexed signal have a constant peak value. Similarly, G_(0,2)is denoted as an AGC control setting level which causes the amplifier 2to have a flat gain vs wavelength characteristic in which theoutput-spectra at the respective wavelengths of the multiplexed signalhave a constant peak value. In order to achieve the above cancellation,the practical AGC control setting levels G₁ and G₂ of the amplifiers 1and 2 are set so that G₁≧G_(0,1) and G₂≦G_(0,2). In this case, as willbe described later with reference to FIG. 2, the amplifiers 1 and 2 canhave gain vs wavelength characteristics that can be compensated by thecombination thereof. For example, the gain of the amplifier 1 at awavelength is large, while the gain of the amplifier 2 at the samewavelength as described above is small. Hence, the total gain obtainedby the amplifiers 1 and 2 can be maintained at a constant (flat) level.By combining the two amplifiers together as described above, it ispossible for the multi-wavelength light amplifier to have nowaveform-dependence of the gain thereof.

The above waveform-dependence of the gain can be maintained at aconstant level irrespective of a variation in the input power by meansof the feedback loop including the light splitting coupler 12, thephotodiode 13, the ALC circuit 14 and the variable attenuator 11. Thesplit light from the beam splitting coupler 12 is applied to thephotodiode 13, which generates an electric signal corresponding to thelight level. The above electric signal is applied to the variableattenuator 11, and the amount of attenuation caused therein is varied onthe basis of the light level detected by the photodiode 13. In thismanner, the light output level of the second-stage amplifier 2 can bemaintained at a constant level. The variable attenuator 11 may be formedby using a Faraday rotator or the electro-optical effect of a lithiumniobate (LiNbO₃) crystal.

The amplifiers 1 and 2 are pumped forward by the pump sources 9 ₁ and 9₂. Alternatively, it is possible to pump the amplifiers 1 and 2backward. It is also possible to pump the amplifiers 1 and 2 forward andbackward.

The light amplifier shown in FIG. 1 is capable of amplifying all thewavelengths to be multiplexed so that the light amplifier does not havethe wavelength-dependence of the gain, which is not changed due to avariation in the power of the input light. If some wavelengths are notused or only some wavelengths are used, a filter (not shown) having acorresponding wavelength characteristic may be placed before thephotodiode 4 ₁ (4 ₃) or 4 ₂ (4 ₄) of the first-stage (second-stage)amplifier 1 (2) or both thereof.

FIG. 2 is a diagram of the operation of a multi-wavelength lightamplifier according to a second embodiment of the present invention. Thesecond embodiment has the same configuration as shown in FIG. 1.According to the second embodiment of the present invention, the opticalfibers 7 and 8 are erbium-doped (Er-doped) optical fibers, which areexamples of rare-earth-element doped optical fibers. Normally, alumina(Al₂O₃) is added to the Er-doped optical fibers at a high concentrationlevel. In this regard, the Er-doped optical fiber may be called aco-doped optical fiber. The Er-doped optical fiber, has a substantiallylinear gain vs wavelength characteristic in an amplifying band about1550 nm, as shown in FIG. 2.

Part (a) of FIG. 2 shows a gain vs wavelength characteristic obtained inthe amplifying band about 1550 nm when the exciting rate is relativelyhigh, and part (b) of FIG. 2 shows a gain vs wavelength characteristicobtained in the amplifying band about 1550 nm when the exciting rate isrelative low. The characteristics shown in parts (a) and (b) of FIG. 2are due to the characteristics of absorption/emission of Er ions in theEr-doped optical fiber with alumina added thereto at a highconcentration level. The horizontal axes of the parts (a), (b) and (c)of FIG. 2 denote the wavelength, and the vertical axes thereof denotethe gain of the Er-doped optical fiber.

As shown in part (a) of FIG. 2, in the amplifying band about 1550 nm,the fiber has a relatively high gain on the short-wavelength side, and arelatively low gain on the long-wavelength side. In other words, as thewavelength becomes shorter, the gain becomes higher. As shown in part(b) of FIG. 2, in the amplifying band about 1550 nm, the fiber has arelatively high gain on the long-wavelength side, and a relatively lowgain on the short-wavelength side. In other words, as the wavelengthbecomes longer, the gain becomes higher.

According to the second embodiment of the present invention, theEr-doped fiber 7 of the first amplifier 1 is long enough to increase theexciting rate and obtain the characteristic shown in part (a) of FIG. 2.The Er-doped fiber 8 of the second amplifier 1 is short enough todecrease the exciting rate and obtain the characteristic shown in part(b) of FIG. 2. Generally, when the pumping of the Er-doped fiber isincreased, the gain vs wavelength characteristic is changed from part(b) of FIG. 2 to part (a) through part (c).

The linear gain slope characteristic of the first-stage amplifier 1 andthat of the gain characteristic of the second-stage amplifier 2 arecanceled by the combination of the amplifiers 1 and 2, so that a flatgain vs wavelength characteristic (a spectrum characteristic having aconstant gain) as shown in part (c) of FIG. 2 can be obtained.

It is preferable for the first-stage amplifier 1 to be a low noisefigure. In this regard, the Er-doped fiber 7 of the first-stageamplifier is used at a relatively high exciting rate. In this case, theexciting efficiency is not high. The Er-doped fiber 8 is used at arelatively low exciting rate. Hence, it is possible to improve theexciting efficiency of the second-stage amplifier 1. This contributes toreducing energy consumed in the second-stage amplifier 2.

The following data has been obtained through an experiment in which themulti-wavelength light amplifier was actually produced. The lightamplifier produced in the experiment was designed to amplify fourwavelengths (1548 nm, 1551 nm, 1554 nm, 1557 nm). The light input levelused in the experiment was selected so as to fall within the range of−25 dBm through—15 dBm. The gain and the gain tilt of the first-stageamplifier 1 were respectively set to 20 dB and 1.5 dB at a maximum powerof the exciting light equal to −160 mW (980 nm). The second-stageamplifier 2 was adjusted so as to produce, for each channel, the lightoutput equal to +7 dBm at a maximum power of the exciting light equal to−100 mW (1480 nm). In this case, the multi-wavelength light amplifierhas a maximum noise figure of 5.6 dB and a maximum gain tilt of 0.2 dB.

FIG. 3 is a block diagram of a multi-wavelength light amplifieraccording to a third embodiment of the present invention. In FIG. 3,parts that are the same as those shown in FIG. 1 are indicated by thesame reference numbers. The light amplifier shown in FIG. 3 has anoptical filter 15 for compensating for a wavelength characteristic, aswill be described below. The optical filter 15 is provided between thevariable attenuator 11 and the input side of the second-stage amplifier2.

FIG. 4 is a diagram showing the operation of the light amplifier shownin FIG. 3. More particularly, part (a) of FIG. 4 shows a gain vswavelength characteristic of the first-stage amplifier 1 shown in FIG.3; and part (b) thereof shows a gain vs wavelength characteristicobtained by the combination of the first-stage amplifier 1 and theoptical filter 15. Part (c) of FIG. 4 shows a gain vs wavelengthcharacteristic of the second-stage amplifier 2 shown in FIG. 4, and part(d) shows a total gain vs wavelength characteristic of the whole lightamplifier shown in FIG. 3.

The configuration of the first-stage amplifier 1 shown in FIG. 3 is thesame as that of the amplifier 1 shown in FIG. 1. The configuration ofthe second-stage amplifier 2 shown in FIG. 3 is the same as that of theamplifier 2 shown in FIG. 3.

The optical filter 15 emphasizes the gain vs wavelength characteristicof the first-stage amplifier 1. As shown in parts (a) and (b) of FIG. 4,the gain for the short wavelengths is particularly emphasized. In otherwords, the linear gain slope of the characteristic shown in part (a) ofFIG. 4 is increased by the optical filter 15. The characteristic of thesecond-stage amplifier 2 shown in part (c) of FIG. 4 compensates for thecharacteristic shown in part (b) thereof, so that the flat gaincharacteristic shown in part (d) of FIG. 4 can be finally obtained.

It will be noted that the exciting rate necessary to obtain thecharacteristic shown in part (c) of FIG. 4 is lower than that necessaryto obtain the characteristic shown in part (b) of FIG. 2. In otherwords, the exciting efficiency of the characteristic shown in part (c)of FIG. 4 is higher than that of the characteristic shown in part (b) ofFIG. 2. Hence, the second-stage amplifier 2 shown in FIG. 3 consumes asmaller amount of energy than that shown in FIG. 1. In other words, ifthe second-stage amplifier 2 shown in FIG. 3 consumes the same amount ofenergy as that shown in FIG. 1, the multi-wavelength light amplifiershown in FIG. 3 can output a larger amount of power than that shown inFIG. 1.

Since the first-stage amplifier 1 has the characteristic shown in part(a) of FIG. 4, it is a low noise figure. The characteristic of thefirst-stage amplifier 1 is emphasized by the optical filter 15, and theexciting efficiency thereof may be improved.

The variable attenuator 11 shown in FIG. 3 is controlled in the samemanner as that shown in FIG. 1 as has been described previously. Inshort, the variable attenuator 11 maintains the level of the outputlight of the second-stage amplifier 1 at the predetermined constantlevel.

FIG. 5 is a block diagram of a multi-wavelength light amplifieraccording to a fourth embodiment of the present invention. In FIG. 5,parts that are the same as those shown in the previously describedfigures are given the same reference numbers. The configuration shown inFIG. 5 differs from that shown in FIG. 3 in that the optical filter 15shown in FIG. 5 is provided between the output side of the second-stageamplifier 2 and the beam splitting coupler 12.

FIG. 6 is a diagram showing the operation of the light amplifier shownin FIG. 5. More particularly, part (a) of FIG. 6 shows a gain vswavelength characteristic of the first-stage amplifier 1 shown in FIG.5, and part (b) thereof shows again vs wavelength characteristic of thesecond-stage amplifier 2 shown in FIG. 5. Part (c) of FIG. 5 is a gainvs wavelength characteristic obtained by the combination of thefirst-stage amplifier 1 and the second-stage amplifier 2. Part (d) ofFIG. 6 shows a total gain vs wavelength characteristic of the wholelight amplifier shown in FIG. 5.

The configuration of the first-stage amplifier 1 shown in FIG. 5 is thesame as that of the amplifier 1 shown in FIGS. 1 and 3. Theconfiguration of the second-stage amplifier 2 shown in FIG. 5 is thesame as that of the amplifier 2 shown in FIGS. 1 and 3.

The optical filter 15 has a gain vs wavelength characteristic whichcompensates for that shown in part (b) of FIG. 2. As shown in parts (a)and (b) of FIG. 6, the characteristic of the second-stage amplifier 2 ispumped so as to have an emphasized gain vs wavelength characteristic, ascompared to that of the first-stage amplifier 1. In the emphasizedcharacteristic, the gain for the long wavelengths is particularlyemphasized. In other words, the linear gain slope of the characteristicshown in part (b) of FIG. 6 is greater than that shown in part (a)thereof although the linear gain slopes shown in parts (a) and (b)thereof are oriented in different directions. The combination of thefirst-stage amplifier 1 and the second-stage amplifier 2 results in thecharacteristic shown in part (c) of FIG. 6. It is not required that thefirst-stage amplifier 1 and the second-stage amplifier 2 havecharacteristics of such a difference which can be completely canceled bythe combination thereof.

The optical filter 15 shown in FIG. 5 has a gain vs wavelengthcharacteristic which compensates for the characteristic shown in part(c) of FIG. 6. Thus, the total characteristic is as shown in part (d) ofFIG. 6.

It will be noted that the exciting rate necessary to obtain thecharacteristic shown in part (b) of FIG. 6 is lower than that necessaryto obtain the characteristic shown in part (b) of FIG. 2. In otherwords, the exciting efficiency of the characteristic shown in part (b)of FIG. 6 is higher than that of the characteristic shown in part (b) ofFIG. 2. Hence, the second-stage amplifier 2 shown in FIG. 5 consumes asmaller amount of energy than that shown in FIG. 1. In other words, ifthe second-stage amplifier 2 shown in FIG. 5 consumes the same amount ofenergy as that shown in FIG. 1, the multi-wavelength light amplifiershown in FIG. 5 can output a larger amount of power than that shown inFIG. 1.

The variable attenuator 11 shown in FIG. 5 is controlled in the samemanner as that shown in FIG. 1 as has been described previously. Inshort, the variable attenuator 11 shown in FIG. 5 maintains the level ofthe output light of the second-stage amplifier 1 at the predeterminedconstant level.

The optical filter 15 used in FIG. 3 or FIG. 5 may be a conventionalcoupler of a melting attachment type. By adjusting the wavelength periodof the coupler, it is possible to use the coupler as a gain tiltingfilter. For example, the optical filter 15 shown in FIG. 5 has a gaintilt equal to approximately 3 dB in order to obtain the flat gaincharacteristic shown in part (d) of FIG. 6.

A description will now be given of a multi-wavelength light amplifieraccording to a fifth embodiment of the present invention. Thisembodiment is intended to obtain the same function as the configurationshown in FIG. 3 without the optical filter 15 shown therein. In otherwords, the light amplifier according to the fifth embodiment isconfigured as shown in FIG. 1, nevertheless it has the function of thelight amplifier shown in FIG. 3.

According to the fifth embodiment of the present invention, the beamsplitting coupler 5 ₂ is replaced by a beam splitting coupler 21 shownin FIG. 7A, which has a transparent rate vs wavelength characteristic asshown in FIG. 7B. In FIG. 7A, a pump source 22 which corresponds to thepump source 9 ₂ is coupled to the beam splitting coupler 21. In FIG. 7B;symbol λ_(p) denotes the wavelength of the pump light emitted from thesource 22. Symbol λ_(s) denotes the central wavelength of themultiplexed light signal. Symbols λ_(s1) and λ_(sn) are wavelengthswhich define the band of the multiplexed light signal. A solid lineshown in FIG. 7B denotes a characteristic used for communications. Twodot lines are obtained by shifting the solid line. As indicated by thesolid line, the beam splitting coupler 21 functions to pass themultiplexed signal light and prevent the pump light in the forwarddirection.

By shifting the solid line toward the short-wavelength side as indicatedby character A in FIG. 7B, the characteristic curve of the transparentrate has a slope in the band defined by the wavelengths λ_(s1) andλ_(sn). In this case, the highest transparent rate can be obtained atthe shortest wavelength λ_(s1), and the lowest transparent rate can beobtained at the longest wavelength λ_(sn). This characteristiccorresponds to the characteristic of the optical filter 15 used in theconfiguration shown in FIG. 3. With the above configuration, themulti-wavelength light amplifier according to the fifth embodiment ofthe present invention has the same advantages as those of the lightamplifier shown in FIG. 3.

The beam splitting coupler 21 can be applied to the first-stageamplifier 1 instead of the second-stage amplifier 2. In this case, theEr-doped optical fiber 7 of the first-stage amplifier 1 is pumpedbackward by the pump source 22 because the optical filter 15 shown inFIG. 3 is placed on the output side of the Er-doped optical fiber 7.

A description will now be given of a multi-wavelength light amplifieraccording to a sixth embodiment of the present invention. Thisembodiment is intended to obtain the same function as the configurationshown in FIG. 5 without the optical filter 15 shown therein. In otherwords, the light amplifier according to the sixth embodiment isconfigured as shown in FIG. 1, nevertheless it has the function of thelight amplifier shown in FIG. 5.

In the sixth embodiment of the present invention, the pump source 9 ₂shown in FIG. 1 is replaced by the pump source 22 shown in FIG. 7Ahaving the transparent rate characteristic indicated by B shown in FIG.7B in such a way that the Er-doped optical fiber 8 is pumped backward bythe pump source 22. This is because the optical filter 15 shown in FIG.5 is placed on the output side of the Er-doped optical fiber 8 shown inFIG. 5.

By shifting the solid line shown in FIG. 7B toward the long-wavelengthside as indicated by character B, the characteristic carve of thetransparent rate has a slope in the band defined by the wavelengthsλ_(s1) and λ_(sn). In this case, the highest transparent rate can beobtained at the longest wavelength λ_(sn), and the lowest transparentrate can be obtained at the shortest wavelength λ_(s1). Thischaracteristic corresponds to the characteristic of the optical filter15 used in the configuration shown in FIG. 5. With the aboveconfiguration, the multi-wavelength light amplifier according to thesixth embodiment of the present invention has the same advantages asthose of the light amplifier shown in FIG. 5.

It will be noted that the above-mentioned third through sixthembodiments of the present invention may be combined appropriately.

FIG. 8 is a multi-wavelength light amplifier according to a seventhembodiment of the present invention. In FIG. 8, parts that are the sameas those shown in the previously described figures are given the samereference numbers. The light amplifier shown in FIG. 8 has asecond-stage light amplifier 2A having a configuration different fromthe above-mentioned second-stage light amplifier 2.

More particularly, the second-stage amplifier 2A has an automatic powercontrol (APC) circuit 10. The APC circuit 10 monitors and controls thepump light emitted from the pump source 9 ₂, so that the pump light canbe emitted at a predetermined constant level. As has been describedpreviously, the variable attenuator 11 functions to maintain theamplified light output by the second-stage amplifier 2 at thepredetermined constant level. Hence, even by the automatic power controlof the pump light directed to maintaining the pump light at the constantlevel, it is possible to maintain the output light of the second-stageamplifier 2A at the predetermined constant level even if the power ofthe light input signal fluctuates.

The first-stage amplifier 1 shown in FIG. 8 has a gain vs wavelengthcharacteristic as shown in part (a) of FIG. 2, and the second-stageamplifier 2A shown in FIG. 8 has a gain vs wavelength characteristic asshown in part (b) of FIG. 2.

The second stage amplifier 2A does not need the couplers 3 ₃ and 3 ₄,and the photodiodes 4 ₃ and 4 ₄. Hence, the second-stage amplifier 2A issimpler than the second-stage amplifier 2, so that down-sizing of thelight amplifier can be facilitated.

FIG. 9 is a block diagram of a multi-wavelength light amplifieraccording to an eighth embodiment of the present invention. In FIG. 9,parts that are the same as those shown in the previously describedfigures are given the same reference numbers. The configuration shown inFIG. 9 differs from the configuration shown in FIG. 1 in that thevariable attenuator 11 shown in FIG. 9 is provided on the output side ofthe second-stage amplifier 2. Thus, the variable attenuator 11attenuates the output light signal of the second-stage amplifier 2 sothat it can be maintained at the predetermined constant level.

It will be noted that in the configuration shown in FIG. 1, theattenuated light signal from the variable attenuator 11 is amplified bythe second-stage amplifier 2. On the other hand, in the configurationshown in FIG. 9, the variable attenuator 11 attenuates the light outputsignal of the second-stage amplifier 2. Hence, the second-stageamplifier 2 shown in FIG. 9 needs a much larger amount of energy of thepump light than that used in the configuration shown in FIG. 1. However,except for the above, the light amplifier shown in FIG. 9 has the sameadvantages as the configuration shown in FIG. 1. For example, the lightamplifier shown in FIG. 9 has a low noise figure because an increase inloss of the gain does not occur between the first-stage amplifier 1 andthe second-stage amplifier 2.

It will be noted that the first-stage and second-stage amplifiers 1 and2 (2A) are not limited to the previously described AGC (APC) circuits inorder to obtain the characteristics shown in FIGS. 2, 4 and 6. It ispossible to arbitrarily combine the previously described AGC circuits.Further, it is also possible to employ other AGC circuits or equivalentsthereof, which will be described below as ninth through eleventhembodiments of the present invention. It will be noted that the AGCcircuit of the first-stage circuit can be selected separately from theAGC circuit of the second-stage circuit.

FIG. 10 is a block diagram of a multi-wavelength light amplifieraccording to a ninth embodiment of the present invention, wherein partsthat are the same as those shown in FIG. 1 are given the same referencenumbers. The light amplifier shown in FIG. 10 has a first-stageamplifier 1B and a second-stage amplifier 2B, which are different fromthe amplifiers 1 and 2.

The first-stage amplifier 1B, which has a gain vs wavelengthcharacteristic as shown in part (a) of FIG. 2, has a forward-directionphotodiode 20 ₁, which detects an amplified spontaneous emission (ASE)leaking from the side surface of the Er-doped optical fiber 7. The AGCcircuit 6 ₁ is supplied with the output signal of the photodiode 20 ₁and controls the pump power of the pump source 9 ₁ so that the amplifiedspontaneous emission can be maintained at a predetermined constantlevel. As a result of the AGC control, the gain of the front-stageamplifier 1B can be maintained at the predetermined constant value.

Similarly, the second-stage amplifier 2B, which has a gain vs wavelengthcharacteristic as shown in part (b) of FIG. 2, has a forward-directionphotodiode 20 ₂, which detects the amplified spontaneous emissionleaking from a side surface of the Er-doped optical fiber 8. The AGCcircuit 6 ₂ is supplied with the output signal of the photodiode 20 ₂and controls the pump power of the pump source 9 ₂ so that the amplifiedspontaneous emission can be maintained at a predetermined constantlevel. As a result of the above AGC control, the gain of thesecond-stage amplifier 2B can be maintained at the predeterminedconstant level.

As has been described previously, the variable attenuator 11 providedbetween the first-stage amplifier 1B and the second-stage amplifier 2Bfunctions to maintain the light output level at the predeterminedconstant level.

FIG. 11 is a block diagram of a multi-wavelength light amplifieraccording to a tenth embodiment of the present invention, in which partsthat are the same as those shown in the previously described figures aregiven the same reference numbers. The light amplifier shown in FIG. 11includes a first-stage light amplifier 1C and a second-stage lightamplifier 2C.

The first-stage light amplifier 1C, which has a gain vs wavelengthcharacteristic as shown in part (a) of FIG. 2, includes a WDM coupler 16₁ and a photodiode 17 ₁. The WDM coupler 16 ₁ separates the light in the1530 nm band (ASE) from the light in the 1550 nm band (signal light).The above ASE travels toward the input side of the Er-doped opticalfiber 7 (backward ASE). The photodiode 17 ₁ detects theamplified-spontaneous emission of the Er-doped optical fiber 7. The AGCcircuit 6 ₁ receives the output signal of the photodiode 17 ₁ andcontrols the pump power of the pump source 9 ₁ so that the backward ASEcan be maintained at a predetermined constant level. As a result of theabove AGC control, the gain of the first-stage amplifier 1C can bemaintained at the predetermined constant level.

The second-stage light amplifier 2C, which has a gain vs wavelengthcharacteristic as shown in part (b) of FIG. 2, includes a WDM coupler 16₂ and a photodiode 17 ₂. The WDM coupler 16 ₁ separates the light in the1530 nm band (ASE) from the light in the 1550 nm band (signal light).The above ASE travels toward the input side of the Er-doped opticalfiber 8 (backward ASE). The photodiode 17 ₂ detects the amplifiedspontaneous emission of the Er-doped optical fiber 8. The AGC circuit 6₂ receives the output signal of the photodiode 17 ₂ and controls thepump power of the pump source 9 ₂ so that the backward ASE can bemaintained at a predetermined constant level. As a result of the aboveAGC control, the gain of the second-stage amplifier 2C can be maintainedat the predetermined constant level.

As has been described previously, the variable attenuator 11 providedbetween the first-stage amplifier 1C and the second-stage amplifier 2Cfunctions to maintain the light output level at the predeterminedconstant level.

FIG. 12 is a block diagram of a multi-wavelength light amplifieraccording to an eleventh embodiment of the present invention, in whichparts that are the same as those shown in the previously describedfigures are given the same reference numbers. The light amplifier shownin FIG. 12 includes a first-stage light amplifier 1D and a second-stagelight amplifier 2D.

The first-stage light amplifier 1D, which has a gain vs wavelengthcharacteristic as shown in part (a) of FIG. 2, includes a WDM coupler 5₃ and a photodiode 18 ₁. The WDM coupler 5 ₃ is provided on the outputside of the Er-doped optical fiber 7, and separates the residual pumplight (exciting light) propagated through the fiber 7 from the signallight. The residual pump light separated by the WDM coupler 5 ₃ isapplied to the photodiode 18 ₁, which outputs a corresponding electricsignal to the AGC circuit 6 ₁. Then, the AGC circuit 6 ₁ controls thepump power of the pump source 9 ₁ on the basis of the detected residualpump light so that the residual pump light can be maintained at apredetermined constant level. As a result of the above AGC control, thegain of the first-stage amplifier 1D can be maintained at thepredetermined constant level.

The second-stage light amplifier 2D, which has a gain vs wavelengthcharacteristic as shown in part (b) of FIG. 2, includes a WDM coupler 5₄ and a photodiode 18 ₂. The WDM coupler 5 ₄ is provided on the outputside of the Er-doped optical fiber 8, and separates the residual pumplight (exciting light) propagated through the fiber 8 from the signallight. The residual pump light separated by the WDM coupler 5 ₄ isapplied to the photodiode 18 ₂, which outputs a corresponding electricsignal to the AGC circuit 6 ₂. Then, the AGC circuit 6 ₂ controls thepump power of the pump source 9 ₂ on the basis of the detected residualpump light so that the residual pump light can be maintained at apredetermined constant level. As a result of the above AGC control, thegain of the second-stage amplifier 2D can be maintained at thepredetermined constant level.

As has been described previously, the variable attenuator 11 providedbetween the first-stage amplifier 1D and the second-stage amplifier 2Dfunctions to maintain the light output level at the predeterminedconstant level.

FIG. 13 is a block diagram of a multi-wavelength light amplifieraccording to a twelfth embodiment of the present invention, whereinparts that are the same as those shown in the previously describedfigures are given the same reference numbers. The light amplifier shownin FIG. 13 differs from that shown in FIG. 1 in that the beam splittingcoupler 12 is provided between the variable attenuator 11 and thesecond-stage amplifier 2.

It is possible to maintain the light output of the second-stageamplifier 2 at the predetermined constant level by controlling thevariable attenuator 11 on the basis of the attenuated light output sothat the attenuated light output is maintained at a predeterminedconstant level. In order to realize the above feedback control, thephotodiode 13 detects a split component of the attenuated light outputand the ALC circuit 14 controls the variable attenuator 11 in theabove-described manner.

FIG. 14 is a block diagram of a multi-wavelength light amplifieraccording to a thirteenth embodiment of the present invention, in whichparts that are the same as those shown in the previously describedfigures are given the same reference numbers. The light amplifier shownin FIG. 14 corresponds to a modification of the light amplifier shown inFIG. 13. The light amplifier shown in FIG. 14 has the first-stage lightamplifier 1 and a second-stage light amplifier 2E.

The second-stage light amplifier 2E, which has a gain vs wavelengthcharacteristic as shown in part (b) of FIG. 2, includes a beam splittingcoupler 3 ₄, the photodiode 4 ₄ and an ALc circuit 14 ₂. It will benoted that the second-stage amplifier 2E is simpler than thesecond-stage amplifier 2 shown in FIG. 13. As has been describedpreviously with reference to FIG. 13, the attenuated light output ismaintained at the predetermined constant level. Hence, the operation ofthe second-stage amplifier 2E receiving the attenuated light outputthrough the beam splitting coupler 12 is equivalent to theAGC-controlled operation of the second-stage amplifier. Hence, it ispossible to control the pump power of the pump source 9 ₂ by theautomatic level control performed by the ALC circuit 14 ₂.

FIG. 15 shows a multi-wavelength light amplifier according to afourteenth embodiment of the present invention. This amplifier includesa rejection filter 30 provided between the first-stage amplifier 1 andthe second-stage amplifier 2. The rejection filter 30 prevents the pumplight propagated from the Er-doped optical fiber 7 from passingtherethrough, and improves the exciting-efficiency of the second-stageamplifier 2. The rejection filter 30 can be applied to the otherembodiments of the present invention in the same manner as shown in FIG.15.

The above-described embodiments of the present invention can bearbitrarily combined to provide variations and modifications.

The present invention is not limited to the specifically disclosedembodiments, and variations and modifications may be made withoutdeparting from the scope of the present invention.

1. An optical transmission system comprising: an optical transmittertransmitting a WDM optical signal including a plurality of opticalsignals with different wavelengths; a multi-stage optical amplifierreceiving the WDM optical signal transmitted by the optical transmitter,amplifying the received WDM optical signal and outputting the amplifiedWDM optical signal; and an optical receiver receiving the amplified WDMoptical signal output from the multi-stage optical amplifier, themulti-stage optical amplifier including a first stage optical amplifieramplifying the WDM optical signal received by the multi-stage opticalamplifier, a level controller controlling a power level of the WDMoptical signal amplified by the first stage optical amplifier, a secondstage optical amplifier amplifying the WDM optical signal of which levelis controlled by the level controller, and an optical coupler splittingthe WDM optical signal amplified by the second stage optical amplifierinto a monitor optical signal and the amplified WDM optical signal to beoutput from the multi-stage optical amplifier.
 2. An opticaltransmission system according to claim 1, wherein the level controllerincludes a variable optical attenuator variably attenuating the WDMoptical signal amplified by the first stage optical amplifier andthereby controlling power level of the amplified WDM optical signaloutput from the multi-stage optical amplifier.
 3. An opticaltransmission system according to claim 1, wherein the multi-stageoptical amplifier further includes: a detector detecting the monitor WDMoptical signal and outputting a monitor signal.
 4. An opticaltransmission system according to claim 3, wherein a gain of themulti-stage optical amplifier is controlled in accordance with themonitor signal.
 5. An optical transmission system comprising: a firstoptical transmission line through which a WDM optical signal including aplurality of optical signals with different wavelengths is transmitted;a multi-stage optical amplifier receiving the WDM optical signal fromthe first optical transmission line, amplifying the received WDM opticalsignal and outputting the amplified WDM optical signal; and a secondoptical transmission line through which the amplified WDM optical signaloutput from the multi-stage optical amplifier is transmitted, whereinthe multi-stage optical amplifier includes a front stage opticalamplifier amplifying the received WDM optical signal to produce a frontstage amplified WDM optical signal, a level controller controlling apower level of the front stage amplified WDM optical signal andoutputting a controlled WDM optical signal, a rear stage opticalamplifier amplifying the controlled WDM optical signal and outputting arear stage amplified WDM optical signal, and an optical couplersplitting the rear-stage amplified WDM optical signal into a monitoroptical signal and the amplified WDM optical signal to be output fromthe multi-stage optical amplifier.
 6. An optical transmission systemaccording to claim 5, wherein the level controller includes a variableoptical attenuator variably attenuating the front stage amplified WDMoptical signal and thereby controlling power level of the amplified WDMoptical signal output from the multi-stage optical amplifier.
 7. Anoptical transmission system according to claim 5, wherein themulti-stage optical amplifier further includes: a detector detecting themonitor WDM optical signal and outputting a monitor signal.
 8. Anoptical transmission system according to claim 7, wherein a gain of themulti-stage optical amplifier is controlled in accordance with themonitor signal.
 9. An optical transmission system comprising: an opticaltransmitter transmitting a WDM optical signal including a plurality ofoptical signals with different wavelengths; a multi-stage opticalamplifier amplifying the WDM optical signal transmitted from the opticaltransmitter and outputting the amplified WDM optical signal; and anoptical receiver receiving the amplified WDM optical signal output fromthe multi-stage optical amplifier, wherein the multi-stage opticalamplifier includes a first stage optical amplifier amplifying the WDMoptical signal transmitted from the optical transmitter, a levelcontroller controlling a power level of the WDM optical signal amplifiedby the first-stage optical amplifier, a second stage optical amplifieramplifying the WDM optical signal of which level is controlled by thelevel controller and outputting the amplified WDM optical signal, and anoptical coupler splitting a monitor WDM optical signal off fromamplified WDM optical signal output by the second stage opticalamplifier.
 10. An optical transmission system according to claim 9,wherein the level controller includes a variable optical attenuatorvariably attenuating the WDM optical signal amplified by the first stageoptical amplifier and thereby controlling the power level of theamplified WDM optical signal output from the multi-stage opticalamplifier.
 11. An optical transmission system according to claim 9,wherein the multi-stage optical amplifier further includes: a detectordetecting the monitor WDM optical signal and outputting a monitorsignal.
 12. An optical transmission system according to claim 11,wherein a gain of the multi-stage optical amplifier is controlled inaccordance with the monitor signal.
 13. An optical transmission systemcomprising: an optical transmitter transmitting a WDM optical signalincluding a plurality of optical signals with different wavelengthsthrough a first optical transmission line; a multi-stage opticalamplifier receiving the WDM optical signal from the first opticaltransmission line and amplifying the received WDM optical signal, themulti-stage optical amplifier including a first stage optical amplifieramplifying the received WDM optical signal, a level controllercontrolling a power level of the WDM optical signal amplified by thefirst-stage optical amplifier, a second stage optical amplifieramplifying the WDM optical signal of which level is controlled by thelevel controller and outputting an amplified WDM optical signal, and anoptical coupler splitting a monitor WDM optical signal off from theamplified WDM optical signal output by the second stage opticalamplifier, an optical receiver receiving the WDM optical signalamplified by the multi-stage optical amplifier through a second opticaltransmission line.
 14. An optical transmission system according to claim13, wherein the level controller includes a variable optical attenuatorvariably attenuating the WDM optical signal amplified by the first stageoptical amplifier and thereby controlling the power level of theamplified WDM optical signal output from the second stage opticalamplifier.
 15. An optical transmission system according to claim 13,wherein the multi-stage optical amplifier further includes: a detectordetecting the monitor WDM optical signal and outputting a monitorsignal.
 16. An optical transmission system according to claim 15,wherein a gain of the multi-stage optical amplifier is controlled inaccordance with the monitor signal.
 17. An apparatus comprising: amulti-stage optical amplifier amplifying a WDM optical signal includinga plurality of optical signals with different wavelengths received froman optical transmitter and outputting an amplified WDM optical signal,the multi-stage optical amplifier including a first stage opticalamplifier amplifying the WDM optical signal received from the opticaltransmitter, a level controller controlling a power level of the WDMoptical signal amplified by the first stage optical amplifier, a secondstage optical amplifier amplifying the WDM optical signal of which levelis controlled by the level controller and outputting the amplified WDMoptical signal, and an optical coupler splitting a monitor WDM opticalsignal off from the amplified WDM optical signal output by the secondstage optical amplifier.
 18. An apparatus according to claim 17, whereinthe level controller includes a variable optical attenuator variablyattenuating the WDM optical signal amplified by the first stage opticalamplifier and thereby controlling the power level of the amplified WDMoptical signal multi-stage optical amplifier.
 19. An opticaltransmission system according to claim 17, wherein the multi-stageoptical amplifier further includes: a detector detecting the monitor WDMoptical signal and outputting a monitor signal.
 20. An opticaltransmission system according to claim 19, wherein a gain of themulti-stage optical amplifier is controlled in accordance with themonitor signal.
 21. An apparatus comprising: a multi-stage opticalamplifier amplifying a WDM optical signal including a plurality ofoptical signals with different wavelengths, the multi-stage opticalamplifier including a front stage optical amplifier amplifying the WDMoptical signal to produce a front stage amplified WDM optical signal, alevel controller controlling a power level of the front stage amplifiedWDM optical signal and outputting a controlled WDM optical signal, arear stage optical amplifier amplifying the controlled WDM opticalsignal to produce a rear stage amplified WDM optical signal, and anoptical coupler splitting a monitor WDM optical signal off from the rearstage amplified WDM optical signal.
 22. An apparatus according to claim21, wherein the level controller includes a variable optical attenuatorvariably attenuating the front stage amplified WDM optical signal andthereby controlling the power level of the rear stage amplified WDMoptical signal.
 23. An apparatus according to claim 21, wherein themulti-stage optical amplifier further includes: a detector detecting themonitor WDM optical signal and outputting a monitor signal.
 24. Anapparatus according to claim 21, wherein a gain of the multi-stageoptical amplifier is controlled in accordance with the monitor signal.